Conventionally, in the fields of medicine and biology, a method in which a fluorescent material composed of organic molecules is used as a marker and fluorescence generated by UV irradiation is measured by an optical microscope or a photo detector. Well-known examples of such a method include an antigen-antibody fluorescent method. In this method, an antibody, to which an organic fluorescent body capable of emitting fluorescence is bound, is used. As the antigen-antibody reaction is extremely highly selective, as is often compared to a relationship between a key hole and a key. Therefore, it is possible to identify the location of the antigen based on the distribution of fluorescence intensity.
As another example, there is a fluorescence-utilizing method using so called DNA chips. When this testing method is employed for a purpose of determining the base sequence of an unknown DNA, the scheme thereof is as follows. That is, by reacting what is called DNA chips in which a large number of DNA (DNA fragments) having known base sequences are arranged in spots-like on a substrate, and DNA having an unknown base sequence which is an organic fluorescent body labeled subject to be tested, the base sequence of the subject is determined by analyzing the position, strength and the like of the fluorescent spots on the DNA chips.
However, the aforementioned conventional organic fluorescent body useful for fluorescent labeling has problems, that is, a problem such that the organic fluorescent body is not stable in storage and at the time of measuring fluorescence, and there is a possibility of deterioration.
In order to solve the problems as describe above, there has been proposed a method of using CdSe nanoparticles (“Semiconductor Nanocrystals as Fluorescent Biological Labels” Marcel Bruchez Jr. et al., p2013-2016, SCIENCE Vol. 281, 25 Sep. 1998; “Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection” Warren C. W. Chan and Shuming Nie. P2016-2018, SCIENCE Vol. 281, 25 Sep. 1998). However, in the above-mentioned method, as the excitation light is blue light or UV light, there arises a problem that, when the subject to be analyzed or detected is a living cell or a living tissue, the excitation light damages the analyzing or detecting subject. Further, when the subject to be analyzed or detected is DNA or a protein, there is a possibility that UV light damages molecules. Therefore, in this method, there is a possibility that determination of base sequences and activity sites with high precision is disturbed.
As particles which emit light by excitation light of a longer wavelengths, Si nanoparticles which emits two-photon excitation have been proposed (“Second harmonic generation in microcrystallite films of ultra small Si nanoparticles” APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 25 18 Dec. 2000). However, as this method is based on a mechanism of light emitting by two-photon absorption, there arises problems in that the light emitting efficiency is poor and the detection precision deteriorates and that super fine particles of no larger than 1 nm are necessary and thus the manufacturing process thereof is complicated.